The present invention relates to charge-coupled devices and, more particularly, to a charge-coupled device structure providing improved packing density on a semiconductor chip, and improved performance when used to implement an image sensor and other functions.
In a conventional charge-coupled device (CCD) individual electrodes are typically disposed in linear arrays over a semiconductor surface and separated from the surface by a thin insulating layer. Applying a particular pattern of voltages to the electrodes defines charge storage regions in the form of potential wells in the semiconductor body below the surface. Each such charge storage region is capable of storing a discrete packet of charge which may vary in magnitude. By appropriately changing voltages applied to the electrodes to modify the configuration of the potential wells, the discrete charge packets stored at each charge storage region may be shifted along a linear array into other linear arrays or detected and converted into current or voltage signals at one end of the array. In this manner a linear array of electrodes is operated as a shift register.
Each electrode of a CCD may be thought of as defining a metal-insulator-semiconductor (MIS) capacitor. Depending upon the configuration of impurity regions in the semiconductor body beneath the surface on which the electrodes are formed, the potential wells serving as charge storage regions may either be adjacent to the semiconductor surface or confined below the surface. Devices of the latter type are referred to as "buried-channel" CCDs. The charge is stored in both cases in a sheet-like region parallel to the surface.
A common application of CCDs is in solid-state image sensors. A typical CCD image sensor comprises a two-dimensional array of electrodes formed above a silicon surface and separated from the surface by a thin layer of silicon dioxide. Typically, a buried-channel CCD configuration is used to minimize charge loss due to surface traps.
By applying an appropriate pattern of voltages to the electrodes, a regular array of charge storage regions is established for an exposure interval. During such an interval, charge generated by light impinging in or near each charge storage region is accumulated within the region. Each charge storage region corresponds to a picture element (pixel) of the image sensor, and the charge stored in such region is representative of the light intensity at the corresponding pixel.
Subsequently, during a scanning interval, which is normally much shorter than the exposure interval, the voltages applied to the electrodes are appropriately varied to rapidly shift the charge packets in each charge storage region accumulated during the exposure interval to a particular edge of the array. Typically, charge packets corresponding to the pixels are moved, one row at a time, into respective charge storage regions of an edge-positioned CCD shift register, which provides the charge packets corresponding to successive rows of pixels in serial form. Image sensors using CCDs are in widespread use in video cameras and other imaging devices.
The total semiconductor area covered by an electrode of a CCD determines the size of the charge storage region associated with the electrode, and hence the amount of charge that may be stored in such a region. Although present day integrated circuit fabrication processes can provide electrodes having an area of less than 1.mu..sup.2, signal-to-noise and dynamic range considerations associated with detecting shall charge packets impose a practical limitation on the minimum area of the electrode, which is generally larger than that imposed by the fabrication process. In conventional CCDs where the electrodes are formed entirely on the planar surface of the semiconductor body, the number of electrodes which may be formed in a given surface area (i.e., the packing density) is limited by the minimum area of the electrode, as determined by signal-to-noise and dynamic range considerations. Therefore, the maximum packing density achievable with conventional CCDs is typically less than that which is permitted by the fabrication process. In the case of a CCD image sensor, a limitation on packing density gives rise to a limitation on the resolution of the image sensor for a given area.
Image sensors using conventional CCDs tend to have a low quantum efficiency in the X-ray and far infrared regions of the spectrum, because of the relatively shallow dimension of the charge storage regions in such devices and the relatively small absorption cross sections for photons in those regions of the spectrum. The poor performance of solid-state, X-ray image sensors formed with conventional CCD structures is described in greater detail in the following published papers: Bautz, M. W. et al., "Charge-coupled-device X-ray detector performance model", Optical Engineering, August 1987, p. 757, and Lumb, David H., et al., "X-ray Measurements of Charge Diffusion Effects in EEV Ltd. Charge-Coupled Devices", Optical Engineering, August 1987, p. 773.
One technique which has been used to improve the performance of solid-state X-ray image sensors using conventional CCD structure is to apply a phosphor on the light-receiving surface of the image sensor which converts X-ray photons into photons of an energy for which the semiconductor material has a higher absorption cross section. However, this technique has not been shown to provide an adequate increase in the overall quantum efficiency of the sensor.
Another technique for improving the performance of surface electrode CCD X-ray Image sensors is to use high-resistivity silicon to provide deeper charge storage regions providing a longer path length for the absorption of an X-ray photon. But CCDs are difficult to fabricate using high resistivity material, because certain contaminants unavoidably introduced during processing tend to lower the resistivity of the semiconductor material. Furthermore, the signal voltage levels required to operate a CCD fabricated with high resistivity material becomes quite high, i.e., on the order of 100 volts.
The use of other semiconductor materials such as gallium arsenide having higher absorption cross sections for X-ray photons has been proposed. However, the fabrication of CCDs with such materials has proven to be difficult.
It is apparent from the foregoing that a need exists for a CCD structure which provides higher packing density on a semiconductor chip. Furthermore, a need exists for a solid-state image sensor having improved quantum efficiency in the X-ray and far infrared regions of the spectrum.